Hey chefn58,
Here's the way I would attack this...
1) First hit the vents with a 120Grit sandpaper and get them back down to metal and smooth out as much as possible. Don't worry about etching the vent with the sandpaper at this point. If you have a Dremel tool, you can also use a mini wire bell brush and it will help take it down as well.
2) Once you have it smooth and showing the metal move down to a 180Grit and just go back over each vent to get them even smoother. Try to always sand in one singular direction at this stage rather than swirling it all over the place. You'll see the payoff a step or two down.
3) This step may not even be necessary but I like to finish off with a final sand at 400Grit to get it looking even smoother.
4) Finally hit it with your best available metal polish. Mothers wheel polish, simichrome or any similar product should do the trick. This step takes some elbow grease but you should now be seeing the results of your effort. Again a Dremel with a polishing tip and paste also works great for this.
Good luck man and post up some after photos for us